Somatostatin (SS) is an endogenous peptide that acts as a hormone, neurotransmitter and neuromodulator, as well as a paracrine regulator of neighboring cells. It is present in two forms, SS-14 and SS-28, a tetradecapeptide and a 28 amino acid peptide, which originate in the protein preprosomatostatin and are distributed differently in neuroendocrine and central and peripheral nervous system tissues. There are five subtypes of somatostatin receptors, all G protein-coupled receptors with a high sequence homology.
Despite the commercial availability of octreotide, lanreotide, and vapreotide, a large number of somatostatin analogs have been proposed for use as imaging and/or therapeutic agents to detect and/or treat cancer and other somatostatin-responsive disease states. Analogs with higher affinity to somatostatin receptors (SSTs) and to SST subtypes, in particular to SST2 and SST5 are desirable, such that lower dosages of somatostatin analogs may be administered to obtain a clinical response.